In the field of optical sensors, the use of optical sensors to detect images in the infrared (IR) and other spectra has been known. Optical detectors such as InGaAs (Indium Gallium Arsenide) or other sensors in the IR band and other frequencies have been deployed in a focal plane with silicon readout circuits. Typically, the current output from the detector device has been delivered to an integration capacitor through an injection MOSFET which, in turn, is gated by a buffer to reduce the detector reverse-bias variations which arise due to photocurrent variations. This configuration is referred to as a buffered direct injection (BDI) readout circuit. A BDI circuit configuration according to known implementations is shown in FIG. 1
The output from the BDI portion of those implementations can be delivered to a current mirror, as also shown in FIG. 1. The current mirror is used to either amplify or attenuate the input photocurrents in order to detect daylight-to-starlight scenes. By varying the voltage differences between BIAS and GAIN in FIG. 1, the ratio between Iout and Iin in FIG. 1 can vary by several orders of magnitude. The right-hand portion of the network is used to reset an integration capacitor used to control the timing of the light-gathering interval performed by the BDI or other readout circuits.
However, in known BDI implementations such as that shown in FIG. 1, there are difficulties in the performance of the optical sensing and output generated by the BDI and current mirror circuits. Specifically, and as for instance shown in FIG. 2, the operating characteristics of the current mirror portion of the circuit, when driven by the buffered current generated by the BDI portion of the circuit, can cause significant delay in the settling time of the scene being imaged, particularly when there is a large change in the amount of luminance in the scene.
That is, and as likewise shown in FIG. 2, when a BDI or other readout circuit is capturing an image with significant brightness variation in consecutive scenes, the readout circuit cannot respond to the scene brightness variations within a reasonable time frame. In FIG. 2, a bright scene occurs at frame 0, while at frame 1 the full brightness cannot be displayed. Rather, full brightness can only be displayed at frame 2. This is referred as “frontend” lag in FIG. 2. Then, at frame 2, the bright scene is switched to dark scene, and the display takes much longer time to settle to the dark scenes in an effect displayed in FIG. 2 as “backend” lag. During that type of flare or other transition, the sensor device may not be able to respond quickly enough to the rapid change in overall luminance of the scene to accurately generate output signal values. This image lag is, in part, caused by the charging/discharging of the capacitance of the current mirror portion of the circuit in FIG. 1. The backend lag time constant, τ, can be estimated according to the expression:τ=RCR is the effective impedance at current mirror gate node and can be in the range of several hundreds of giga-Ω in dark scene. A simple estimation of R can be related to the trans-conductance, gm, of Min transistor in FIG. 1. R≈1/gm≈(1.5*kT/q)/Iin where k is Boltzmann constant, T is temperature, q is electron charge and Iin is photocurrent. If T=300° K and photocurrent=0.1 pA, R is almost 400 GΩ. C is the effective capacitance at the current mirror gate node. That is, from the bright scene to the dark scene, the time constant to settle the discharging of current mirror gate node can be many tens of milli-seconds. Since the typical frame time is 33 mS, it is not surprising that backend image lag can last for several frames, which can significantly affect image quality or accuracy, under transitional luminance conditions.
It may be desirable to provide methods and systems for image lag mitigation for buffered direct injection readout circuitry with a current mirror, in which optical sensors can deliver image signals via a BDI portion which in turn feeds a current mirror topology that provides improved response time for changing image conditions.